1. Field of the Invention
This invention relates to couplings of the kind used on articulated vehicles, for connecting the tractor and associated trailer of such vehicles and the like.
2. Prior Art
Couplings of this kind connect the trailer to the tractor by engagement of a kingpin, which depends from the front end of the trailer, in a slot provided in the load bearing plate of a fifth wheel assembly mounted on the tractor, the kingpin being releasably held in the plate and the vertical axis of the kingpin being the axis about which relative angular movement of the tractor and trailer takes place.
Jackknifing is a particular hazard to which vehicles using couplings of the said kind are susceptible. One proposal for overcoming, or at least reducing, the hazard has been to provide a fifth wheel assembly having a load bearing plate rotatably mounted on the tractor and releasably engaged by projecting means on the trailer in such manner that the trailer and the load bearing plate are restrained from rotation with respect to each other, and fitted with braking means or with rams, mounted on the tractor, operable to act upon the load bearing plate to resist rotation of the load bearing plate relative to the tractor.
The use of rams, as opposed to braking means such as a disc and callipers as disclosed in U.S. Pat. No. 3,985,369, restrict the extent to which the load bearing plate may rotate since the ram piston rods soon foul the centre shaft of the load bearing plate.
One proposal for overcoming this problem in a coupling using rams consists, as described in British Application No. A 2,019,332, in providing the load bearing plate with a toothed rim meshing with a toothed segment or a sprocket chain mounted on the tractor, opposite portions of the segment or chain being hinged to the ends of a pair of rams (in the case of the segment) or passing around sprockets rotatably mounted on the rams (in the case of the chain), the rams being mounted on the tractor on opposite sides of the plate axis.
These arrangements are expensive and prone to breakage.